Effect of weather and climatological background on snow damage of forests in Southern Finland in November 1991.

Snow damage to forests in Southern Finland in November 1991 is examined in relation to meteorological conditions. The combined effect of different factors proved to be necessary for severe damage. First, the snow load, in terms of precipitation, should exceed a certain limit. The limit can be set for weak or moderate damage at about 40 mm and for very severe damage at about 60 mm. Secondly, temperature at the time of precipitation should be above 0 °C, so that snow on twigs becomes slightly wet in order that it should attach to twigs during the subsequent period with temperature below 0 °C. On the other hand, temperatures exceeding 0.6 °C prohibit damage by permitting the snow load to fall. Wind speeds exceeding 9 ms–1, as observed 15 m above ground, were strong enough to dislodge the snow which is not attached, and thus reduce the damage. There are few statistics either of snow damage or of the relation between the snow damage and precipitation. However, there is a causal connection between snow damage and heavy snowfalls. Therefore, the regions with a high frequency of heavy snowfalls, as indicated by orographical features and occurrence of thick snow cover, were investigated